556 HEREDITY 



being composed of father chromatin and the other half being com- 

 posed of mother chromatin. This means that the chromosomes 

 contributed to the offspring by each of the parents maintain 

 their individuality in the offspring. In vegetative cell division 

 each chromosome splits longitudinally, and to each new nucleus 

 there is contributed a half of each chromosome. It is obvious 

 that the vegetative cell division tends to distribute the chromatin 

 from both parents equally to the new nuclei. But in the reduc- 

 tion division, as shown in Figure 483, whole chromosomes and 

 not halves are contributed to each new nucleus, and conse- 

 quently the new nuclei resulting from the reduction division 

 receive only half as many chromosomes as the mother cell con- 

 tained. In the reduction division the chromosomes contributed 

 to the daughter nuclei may be only those of the mother parent or 

 only those of the father parent, in which case the daughter nuclei 

 receive only the genes of one of the parents. On the other 

 hand, the daughter nuclei may receive chromosomes of both 

 parents and in different proportions in different divisions. Again 

 cytological studies of the reduction division show that there 

 is a pairing of chromosomes previous to their separation, and 

 there is evidence that each pair consists of a father and a 

 mother chromosome. Now, if we assume that chromosomes 

 pairing carry genes for contrasting characters, then the separa- 

 tion and distribution of the members of each pair to different 

 daughter nuclei should result in the segregation of genes for 

 contrasting characters and in the production of pure gametes. 

 The trouble with this assumption is that a plant or animal has 

 so many more pairs of contrasting characters than chromo- 

 somes, that it is difficult to explain the numerous combinations 

 that occur when many pairs of contrasting characters are taken 

 into account. Despite the fact that there are some things 

 about segregation we are unable to explain by the mechanism 

 of reduction division, it is generally believed that the two phe- 

 nomena are vitally related. 



*The Mendelian Ratio and the Combinations of Gametes. 

 It is possible to account for the Mendelian ratio 1:2:1 by 

 taking into account the probable combinations that may occur 

 among gametes during fertilization. A hybrid forms two kinds 

 of gametes equal in number in respect to a pair of contrasting 

 characters. One kind of sperms and eggs may be represented, 



